1. Technical Field of the Disclosure
The present disclosure relates to shape-memory alloy wire and, in particular, relates to a method of manufacturing wire made of a shape memory alloy which demonstrates wire properties appropriate for in vivo use, as well as medical devices made with such wire.
2. Description of the Related Art
Shape memory materials are materials that “remember” their original shape, and which, after being deformed, return to that shape either spontaneously or by applying heat to raise their temperature above a processing and material related threshold known as the transformation temperature. Materials which are heated to recover shape are commonly referred to in the art as “shape memory” materials, whereas materials which spontaneously recover to a particular shape upon removal of a constraining force are commonly referred to as pseudoelastic materials. Shape memory materials, possessing relatively high transformation temperatures, are commonly used in thermally activated actuation devices, for example in military, automotive or robotic devices whereas pseudoelastic materials are commonly used in highly flexible implantable medical devices.
Pseudoelasticity, sometimes called superelasticity, is a reversible response to an applied stress, caused by a phase transformation between the austenite or parent phase and the martensite or daughter phase of a crystal. FIG. 1 schematically illustrates superelastic behavior in the context of a stress-strain curve for a superelastic material. As illustrated, a superelastic stress-strain curve exhibits a nonlinear correlation between load (i.e., stress) and displacement (i.e., strain). During initial loading across a first change in strain Δε1, stress and strain are linearly correlated, that is, the material exhibits linear elastic behavior. Superelastic behavior is exhibited by the shape of the stress-strain curve across Δε2, which disrupts the end of this linear elastic correlation with a substantial increase in strain with little or no increase in stress. The magnitude of Δε2 can be considered to be the “amount” or level of loading superelasticity exhibited by a material. During this period, phase transformation occurs between austenite and martensite. At the end of the phase transformation, the material may experience a further linear elastic deformation through Δε3. Upon unloading, linear elastic behavior is again exhibited for the initial unloading process, with superelastic behavior occurring across Δε4. The magnitude of Δε4 can be considered to be the “amount” or level of unloading superelasticity exhibited by a material. Notably, the unloading superelasticity quantified by Δε4 is smaller, and occurs and a lower stress, as compared to the loading superelasticity quantified by Δε2. This disparity between loading and unloading behavior is a feature of superelastic materials.
A pseudoelastic material may return to its previous shape after the removal of even relatively high applied strains by heating. For example, even if the secondary or daughter domain boundaries do become pinned, for example due to dislocations associated with plasticity, the material may be reverted to the primary or parent phase by stresses generated through heating.
Pseudoelasticity is generally exhibited in shape memory alloys. Pseudoelasticity and shape memory both arise from the reversible motion of domain boundaries during the phase transformation, rather than just bond stretching or the introduction of defects in the crystal lattice. Examples of shape memory materials include iron-chrome-nickel, iron-manganese, iron-palladium, iron-platinum, iron-nickel-cobalt-titanium, iron-nickel-cobalt-tantalum-aluminum-boron, copper-zinc-aluminum, copper-zinc-aluminum-nickel, copper-aluminum-nickel, and nickel-titanium alloys. Shape memory materials can also be alloyed with other materials including zinc, copper, gold, and iron.
Shape memory materials are presently used in a variety of applications. For example, a variety of military, medical, safety and robotic applications for shape memory materials are known. Medical grade shape memory materials are used for orthodontic wires, guide wires to guide catheters through blood vessels, surgical anchoring devices and stent applications, for example. One shape memory material in wide use, particularly in medical device applications, is a nickel-titanium shape memory material known as “Nitinol”.
Many medical grade shape memory wire products are made of biocompatible implant grade materials including “NiTi” materials. As used herein, “nickel-titanium material”, “nickel-titanium shape memory material” and “NiTi” refer to the family of nickel-titanium shape memory materials including Nitinol (an approximately equiatomic nickel-titanium, binary shape memory material) as well as alloys including nickel and titanium as primary constituents but which also include one or more additional elements as secondary constituents, such as Nitinol tertiary or quaternary alloys (e.g., Nitinol with additive metals such as chromium, tantalum, palladium, platinum, iron, cobalt, tungsten, iridium and gold).
Wire products made of shape memory materials are manufactured by forming a relatively thick piece of hot-worked rod stock from a melt process. The rod stock is then further processed into wires by drawing the rod stock down to a thin diameter wire. During a drawing process, often referred to as a “cold working” process, a wire is pulled through a lubricated die to reduce its diameter. The deformation associated with wire drawing increases the stress in the material, and the stress eventually must be relieved by various methods of heat treatment or annealing at elevated temperatures to restore ductility, thus enabling the material to undergo further cold working to further reduce the wire diameter. Each time the wire is annealed to enable further cold working, the accumulated internal stress—and therefore, the accumulated cold work—is “reset” to a zero level. These iterative processes of cold working and annealing may be repeated several times before a wire of a desired diameter is produced and processing is completed.
Although pseudoelastic wires made in accordance with foregoing processes may demonstrate desirable material properties for in vivo use, such wires are not generally radiopaque, i.e., the material of the wire allows X-rays or other types of radiation to pass through and therefore the wire material is not sufficiently distinguishable from surrounding anatomic structures in X-ray images. Substantial design efforts have been focused on imputing radiopacity to in vivo wire materials, such as by creating composite wires which utilize a radiopaque core material surrounded by a biocompatible sleeve suitable for in vivo use. Where in vivo wires are made radiopaque, X-ray imaging equipment can be used to assess, verify and/or monitor the location of the wire within the patient's body.
A primary constituent material of NiTi wires is nickel, as noted above. In some instances, it may be desirable to avoid the use of nickel for implanted medical devices, thereby minimizing any chance for adverse in vivo reactions in nickel-sensitive patients.